Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties, dimensional stability, etc. The PASs are commonly used in a wide variety of fields such as electrical and electronic equipments and automotive equipments because they can be molded or formed into various kinds of molded or formed products, films, sheets, fibers, etc. by general melt processing processes such as extrusion, injection molding and compression molding.
As a typical production process of a PAS, is known a process in which a sulfur source is reacted with a dihalo-aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone. As the sulfur source, is generally used an alkali metal sulfide, an alkali metal hydrosulfide or a mixture thereof. When the alkali metal hydrosulfide is used as the sulfur source, the alkali metal hydrosulfide is used in combination with an alkali metal hydroxide.
When at least one sulfur source selected from the group consisting of alkali metal sulfides and alkali metal hydrosulfides is reacted with a dihalo-aromatic compound, a great amount of an alkali metal salt such as NaCl is secondarily produced by a desalting condensation reaction. The alkali metal salt adversely affects the electrical properties of the resulting PAS because the salt is an electrolytic component. The PAS is used as, for example, a sealing compound or covering material for electronic parts. When the PAS containing the alkali metal salt is used in these applications, the alkali metal salt corrodes electrodes or wiring of the electronic part, causes disconnection or makes a leakage current great.
There have heretofore been proposed various methods for reducing the content of the alkali metal salt contained in the PAS. For example, Japanese Patent Application Laid-Open No. 55-156342 (Patent Literature 1) proposes a method for extracting PPS powder with heated deionized water over a long period of time. However, the extraction method with the heated water involves a problem that an extraction efficiency of the alkali metal salt is low in addition to the fact that it takes a long time for the extraction.
Japanese Patent Application Laid-Open No. 59-219331 (Patent Literature 2) proposes a method for reducing the content of an alkali metal salt by subjecting PPS to a heat treatment in an aromatic solvent. However, this method requires to conduct the heat treatment of the PPS after polymerization in the aromatic solvent such as diphenyl ether for a long period of time.
In addition, the methods proposed by these Patent Literatures 1 and 2 intend to remove alkali metal salts such as NaCl secondarily produced and cannot reduce the content of a halogen atom such as a chlorine atom bonded to a terminal. More specifically, when a sulfur source is reacted with a dihalo-aromatic compound, a PAS with a halogen atom such as a chlorine atom bonded to a terminal of its polymer chain is formed in addition to the fact that a great amount of an alkali metal salt such as NaCl is secondarily produced by a desalting condensation reaction.
As a method for reducing the content of the bonded chlorine, Japanese Patent Application Laid-Open No. 62-106929 (Patent Literature 3) proposes a method in which PPS is subjected to a heat treatment with a mercapto group-containing compound or an alkali metal salt thereof in a solvent capable of dissolving the PPS. However, this method is not efficient because the PPS is caused to react with the particular reaction reagent at a high temperature over a long period of time after the PPS is washed in a post treatment step after polymerization, and filtering and washing steps are further required.
Japanese Patent Application Laid-Open No. 5-163349 (Patent Literature 4) proposes a process for producing a PAS having a low bonded chlorine content by subjecting a cyclic arylene sulfide oligomer to ring-opening polymerization under heating in the presence of a ring-opening polymerization catalyst. However, this method requires to extract a 7- to 15-mer cyclic phenylene sulfide oligomer from a PPS formed after a polymerization reaction of sodium sulfide with p-dichloro-benzene in an organic amide solvent by Soxhlet extraction using methylene chloride as a solvent. Therefore, it is difficult to apply this method to production on an industrial scale.
On the other hand, there have been proposed various processes for producing a high-molecular weight PAS using various polymerization aids in a production process of a PAS by polymerizing a sulfur source and a dihalo-aromatic compound in an organic amide solvent. For example, Japanese Patent Publication No. 52-12240 (Patent Literature 5) discloses a production process of PPS using a carboxylic acid alkali metal salt as the polymerization aid. Japanese Patent Application Laid-Open No. 59-219332 (Patent Literature 6) discloses a production process of PPS using an alkaline earth metal salt or zinc salt of an aromatic carboxylic acid as the polymerization aid. U.S. Pat. No. 4,038,263 (Patent Literature 7) discloses a production process of PPS using an alkali metal halide as the polymerization aid. Japanese Patent Application Laid-Open No. 1-161022 (Patent Literature 8) discloses a production process of PPS using the sodium salt of an aliphatic carboxylic acid as the polymerization aid. Japanese Patent Publication No. 63-33775 (Patent Literature 9) discloses a production process of a PAS using water as the polymerization aid.
The amount and time of these polymerization aids added, the temperature of a polymerization reaction system, etc. are controlled, whereby a phase-separated state that a concentrated formed polymer phase and a dilute formed polymer phase are present in a mixed state can be created in a liquid phase within the polymerization reaction system. When the polymerization reaction is continued in such a phase-separated state, the formation of a high-molecular weight PAS is caused to progress, and moreover the high-molecular weight PAS can be obtained in the form of granules by slowly cooling the polymerization reaction system after the polymerization reaction. Therefore, these polymerization aids are called phase separation agents.
More specifically, when at least one sulfur source selected from the group consisting of alkali metal sulfides and alkali metal hydrosulfides is reacted with a dihalo-aromatic compound, a desalting condensation reaction between the monomers is caused to rapidly progress to increase the conversion of the dihalo-aromatic compound. However, the polymer in this state is low in melt viscosity (molecular weight) and is in a stage of the so-called prepolymer. When a phase-separated state that a concentrated formed polymer phase and a dilute formed polymer phase are present in a mixed state is created in a liquid phase within the polymerization reaction system in the presence of the phase separation agent, the concentrated formed polymer phase is dispersed in the dilute formed polymer phase by stirring, and a condensation reaction between the prepolymers is caused to efficiently progress in the concentrated phase. As a result, the formation of a high-molecular weight polymer is advanced.
In the granular, high-molecular weight PAS, impurities such as the secondarily formed alkali metal salt and oligomers are easily removed therefrom by washing. At present, the granular, high-molecular weight PAS is washed by a combination of water washing, washing with an organic solvent, acid washing, etc., whereby a PAS substantially containing no alkali metal salt such as NaCl comes to be obtained. The granular, high-molecular weight PAS sufficiently washed does substantially not produce ash even when it is burnt.
However, it is extremely difficult to reduce the content of the bonded halogen atom such as a chlorine atom bonded to a terminal even by the production process of the PAS comprising such a phase-separation polymerization step. On the other hand, the requirement of halogen-free regulation on the whole polymer product from environmental groups is heightened in addition to the requirement of halogen-free regulation on PASs from electronic part makers, and PAS makers bear the responsibility of satisfying these requirements.
By the way, Japanese Patent Publication No. 5-11128 (Patent Literature 10) discloses a process in which upon production of a PAS by reacting an alkali metal sulfide and a dihalo-aromatic compound in a polar organic solvent, a monohalo or polyhalo-substituted organic compound (however, excluding compounds, on the aromatic ring of which a halogen or halogens are directly substituted), is added to conduct the reaction. However, the process disclosed in Patent Literature 10 intends to raise the melt crystallization temperature of the resulting PAS, and does not intends to produce a PAS whose content of a terminal halogen group has been reduced.
Specifically, in the process disclosed in Examples of Patent Literature 10, the monohalo or polyhalo-substituted organic compound is only added into the polymerization reaction system at the beginning of a polymerization reaction or at the time final-stage polymerization (phase-separation polymerization) has been substantially completed. However, it is difficult to obtain a PAS whose content of a terminal halogen group has been reduced by such a process.